Kinetic energy Definition and 264 Discussions

In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body when decelerating from its current speed to a state of rest.
In classical mechanics, the kinetic energy of a non-rotating object of mass m traveling at a speed v is





{\displaystyle {\begin{smallmatrix}{\frac {1}{2}}mv^{2}\end{smallmatrix}}}
. In relativistic mechanics, this is a good approximation only when v is much less than the speed of light.
The standard unit of kinetic energy is the joule, while the English unit of kinetic energy is the foot-pound.

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  1. J

    Speed or turning point using energy

    Homework Statement Please look at the attached screenshot. This problem is really confusing for me and I can't seem to make much sense out of it. Homework Equations Ei = Ef The Attempt at a Solution As you can see, I did get (a). (The other checkmarks, I guessed — there were only two...
  2. A

    How much work needs to be done on a piano to move it into the bed of a truck?

    Homework Statement Movers must push a piano onto a truck, the bed of which is a height 1.35 m above the ground. To do this they will use a frictionless ramp. If the piano has a mass of 1806.0 kg and the movers push it up the slope at a constant velocity, how much work do they need to do on it...
  3. M

    Elastic collision with particles, find the kinetic energy

    Homework Statement A proton strikes a stationary alpha particle (4He nucleus) head-on. Assuming the collision is completely elastic, what fraction of the proton’s kinetic energy is transferred to the alpha particle? Homework Equations Pi = Pf Ki = Kf The Attempt at a Solution Tried finding...
  4. sush

    I Why E=mc^2 is different from E=1/2 mv^2?

    Why kinetic energy is ½ m v2? Why it is different from Einstein’s equation for Energy E= m c2?
  5. F

    Distance traveled with linearly increasing kinetic energy

    Homework Statement An object of mass 50 kg gains 20,000,000 joules every second. Devise formulae to find the distance covered at any given point in time, and the time necessary to cover a certain distance. Homework Equations The Attempt at a Solution E = 20,000,000 * t V = (E * 2/50)1/2...
  6. Ennio

    Formulating the kinetic energy of an object - helical motion

    Hi guys, I need your support to formulate the kinetic energy of an object: - having mass m [Kg] - rotating with angular velocity o [rad/sec] referred to an axis t [m] distant (and parallel) to the symmetry axis of the object - moving along the direction of its symmetry axis with a costant...
  7. Exath

    Rotation of Rigid Bodies: Rotating stick with disc on top

    Homework Statement [/B] A thin cylindrical rod with the length of L = 24.0 cm and the mass m = 1.20 kg has a cylindrical disc attached to the other end as shown by the figure. The cylindrical disc has the radius R = 8.00 cm and the mass M 2.00 kg. The arrangement is originally straight up...
  8. A

    Do electromagnetic waves have potential and kinetic energy

    Do electromagnetic waves have potential and kinetic energy like springs, strings, etc. If so how are they calculated, inter-related? What is the total energy? Are the energies fluctuating over time? I am a physics hobbyist so generally the first answers should come with the least mathematics...
  9. I

    Find the launch speed of a ball in a spring mechanism

    Homework Statement I have a question asking me to find the launch speed of a ball (mass 0.39kg) when released by a spring mechanism made of 2 springs each with force constant 25Nm^-2. they are pulled back 12 cm. the ball is initially at rest. Homework Equations [/B] v^2=u^2+2as f=ma f=kx...
  10. negative

    Relation between boltsmann/gas constant and temperature

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  11. A

    Kinetic energy of a bouncing ball

    Homework Statement I'm trying to find the coefficient of restitution (COR) and energy loss upon impact of a bouncing ball, and I'm trying to understand how these relate to each other. Homework Equations COR=sqrt(2gh/2gH) suvat equations (possibly) 1/2mv^2=mgH (using m=0.0585kg, g=9.8) energy...
  12. J

    Mechanical energy for planet in elliptical orbit around star

    Homework Statement A planet is in an elliptical orbit around a star. Which of the following best represents the mechanical energy E_planet of just the planet and the mechanical energy Es_tar-planet of the star-planet system as functions of time for one complete orbit? Homework Equations...
  13. J

    Which sphere reaches the bottom of the inclined plane first

    Homework Statement Two spheres are placed side by side on an inclined plane and released at the same time. Both spheres roll down the inclined plane without slipping. (a) Using FBD, explain what force provides the torque allowing the sphere to roll down the inclined plane. (b) Which sphere...
  14. M

    Conservation of momentum - with understanding

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  15. M

    Angular momentum with a large wooden turntable

    Homework Statement Homework Equations Li = Lf L = I*omega K = (1/2)*(I)*(omega)^2 The Attempt at a Solution [/B] Given that there are no non-conservative forces in action, I am assuming that the two kinetic energies should be equal. However, as shown by my work above, the two values...
  16. E

    Finding the Kinetic Energy using the mass, radius and initial KE

    Information Given:Zero, a hypothetical planet, has a mass of 4.2 x 1023 kg, a radius of 2.8 x 106 m, and no atmosphere. A 10 kg space probe is to be launched vertically from its surface. Question: (a) If the probe is launched with an initial kinetic energy of 5.0 x 107 J, what will be its...
  17. D

    Change in the Kinetic Energy of a System

    The question is to find the change in kinetic energy of a system. the system in use is a pully system with a wooden block on a flat surface attached to a 100g mass. I have calculated force of friction between the block and desk, and using a ticker timer, I have individual speeds at every 0.1 of...
  18. A

    How close will body approach fixed point charge?

    Homework Statement Charged sphere with a mass of 15 mg and charge 2 nC moves with a speed of 15 cm/s towards a fixed point charge of 3 nC. How close will sphere approach charge? Homework Equations K=(1/2)*mv2 U=k*(Q1Q2/r) The Attempt at a Solution So I am not sure I approached correctly but...
  19. E

    Momentum and Kinetic Energy

    Homework Statement On a frictionless, horizontal air table, puck A (with mass 0.250 kg ) is moving toward puck B (with mass 0.360 kg ), that is initially at rest. After the collision, puck A has a velocity of 0.125 m/s to the left, and puck B has velocity 0.655 m/s to the right. I already...
  20. T

    Kinetic energy of a solid versus a liquid at the same temperature (through a phase change)

    Hi. In all of my question, I mean degrees celsius when I say degree. Think we have a 0 degree ice. we give some energy to it (by a heater) and all of it get into a 0 degree water. How does the kinetic and potential energy of the molecules change? In the book in which I saw this question, it...
  21. K

    Lifting an object, kinetic energy, work and forces

    Homework Statement From a physics multiple choice test: "A car is raised to a height of 10 meters. The car is at rest both before and after it has been lifted. The mass of the car is 900 kg. The total work that has been done on the car during the move is" (The right answer is: zero). Wnet...
  22. S

    Help me find my error in a relativistic kinetic energy calculation

    Hi everyone. Given: an asteroid with the mass of 50,000,000 kg, which is moving with the velocity of oh-my-god particle -- 99.99999999999999999999951% of c. Due to relativistic effects, its total kinetic energy will be 1.44 E+36 Joules (Lorentz factor = 3.2 E+11). A hypothetical particle...
  23. G

    B Coefficient of Restitution

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  24. E

    Displacement of an object with certain Ek stopped by Frictional Force

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  25. How to Find the Potential Energy of a Particle # 9 - YouTube

    How to Find the Potential Energy of a Particle # 9 - YouTube

    How can one find the potential energy of a particle. The derivation of PE can be done by the fact that change in PE of a particle is equal to negative of work done by a force on the particle. This combined with Kinetic energy work theorem gives us an equation on conservation of mechanical energy.
  26. G

    Energy required to accelerate itself

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  27. S

    Work, Energy, and Power

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  28. R

    I Integration by parts

    Hi, I've been following a derivation of relativistic kinetic energy. I've seen other ways to get the end result but I'm interested in finding out where I've gone wrong here: I'm struggling with integrating by parts. The author goes from...
  29. K

    B Equation dealing with energy?

    What is ΔEp = ΔEk + W I have come across this formula, but i haven't seen it before. What is it used for?
  30. M

    Finding the speed of the block for one and two springs

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  31. K

    Side kick on a falling object, mechanical energy and work

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  32. P

    Time of Electron/Proton to Radiate Away Half of Their Energy

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  33. Jamie_Pi

    Starting height of marble rolling around a loop the loop

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  34. Anonymous123451234

    Inelastic collision, determine ratio KEf/KEi

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  35. A

    Work and Kinetic Energy

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  36. M

    Why does kinetic energy depend on the frame of reference?

    I'm having this discussion with my engineering peers: A ball is sitting on top of a train traveling at, say, 10m/s. The ball has mass of 2kg, for simplicity. The ball's kinetic energy KE relative to ground zero is 1/2 m v^2, or 100J. A person riding on the train picks up the ball and tosses...
  37. G

    Electric Potential Energy - Final Velocity

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  38. L

    Object on a vertical Ring with spring attached

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  39. Irfan Nafi

    Which ball has more energy halfway through the track?

    2 frictionless tracks have equal length bumps with one going up and the other down. Halfway through the track which one has more energy? Assuming that they started at the same height, I thought that they had the same amount of energy, but since Ball B ends up having a greater velocity at the...
  40. A

    GPE and escape velocity

    Is it necessary to be the kinetic energy greater than gpe to move ( I don't talk about in orbits) Example : Is it will be harder to move an object has a bigger gpe ( same mass but bigger hight from ground). And thanks.
  41. J

    Calculate change in KE and momentum (impulse)

    Homework Statement I have attached the problem. A 14,000 kg tractor traveling north at 21 km/h turns west and travels 26 km/h. Calculate the change in the tractor's a. kinetic energy b. linear momentum (magnitude AND direction) Homework Equations [/B] delta KE = (1/2)mv^2 - (1/2)mvi^2...
  42. J

    Final speed and direction after a collision (elastic+inelastic)

    Homework Statement A billiard ball moves at a speed of 4.00 m/s and collides ELASTICALLY with an identical stationary ball. As a result, the stationary ball flies away at a speed of 1.69 m/s. Determine a. the final speed and direction of the incoming ball after the collision b. the direction...
  43. P

    Significant Figures with Kinetic Energy Formula?

    Homework Statement An object of mass m = 2.3±0.1 kg is moving at a speed of v = 1.25±0.03 m/s. Calculate the kinetic energy (K = 1 /2mv2 ) of the object. What is the uncertainty in K? I am not exactly sure if I used the error equation correctly when I start using Δ(v2). Could someone verify my...
  44. K

    Can anyone help with this magnetism problem?

    A 2.53*10^(-6) C charged particle with kinetic energy of 0.0929 J is fired into a uniform magnetic field of magnitude 0.147 T. If the particle moves in a circular path of radius 3.38 m, determine its mass in kg. (answer should be within 2*10^-14 kg) The formula r=mv/qb comes to mind where r is...
  45. G

    A Usage of kinetic energy in proof for E=mc^2 Above is a question that I posted on a school help site. This is the answer I got: Can you use this answer to show that the type of calculation that I tried in...
  46. Kiwigami

    Understanding Drag Force against a non-linear Force

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  47. Richardparker800

    Need help with collision and conservation of momentum problem

    Homework Statement Two blocks move along a horizontal frictionless surface and collide head-on with each other. The mass of the first block, moving to the right with speed v is m, and the mass of the second block moving towards the left with speed 3v is 2m. After the collision, the block of...
  48. phasacs

    Why isn't Kinetic Energy always equal to Potential Energy?

    K= ∫mvdv = ∫m dx/dt dv = ∫m dx/dv dv/dt dv = ∫m dv/dt dx = ∫Fdx = U => K=U, why isn't this true? If it is, wouldn't that mean that Kinetic Energy is always equal to Potential Energy?
  49. N

    What % of PEg must be converted in KE in order to double v?

    Homework Statement What % of PEg must be converted in KE in order to double the speed of a pendulum? So I have spent a couple of days trying to figure out this problem (it's a lab), and I can't seem to figure it out. I have asked my physics teacher about it, and he said the answer to the...
  50. Y

    Help in Homework - Kinetic Energy

    Homework Statement Problem: Hi all, I was given this problem as homework and I have almost no clue on how to solve it. I have tried for some time with no luck. Homework Equations We learned about kinetic and potential energy as well as work. W = F*d W = delta KE PE =...